[cmake] Use llvm-lit directory when provided for stand-alone build

[mgorny]

Summary

After the recent lit test changes, clang attempts to run its tests
via llvm-lit by default. However, the llvm-lit binary is not present
when performing stand-alone build resulting in a failure out of the box.

To solve that, add the llvm-lit directory to CMake when performing
a stand-alone build and LLVM sources are provided. This includes
the CMake rules generating the llvm-lit binary and effectively makes
it possible for clang to use it.

cfe/trunk/CMakeLists.txt

if(EXISTS ${LLVM_TOOLS_BINARY_DIR}/FileCheck${CMAKE_EXECUTABLE_SUFFIX}
         AND EXISTS ${LLVM_TOOLS_BINARY_DIR}/count${CMAKE_EXECUTABLE_SUFFIX}
         AND EXISTS ${LLVM_TOOLS_BINARY_DIR}/not${CMAKE_EXECUTABLE_SUFFIX})
       set(LLVM_UTILS_PROVIDED ON)
     endif()
 
     if(EXISTS ${LLVM_MAIN_SRC_DIR}/utils/lit/lit.py)
       # Note: path not really used, except for checking if lit was found
       set(LLVM_LIT ${LLVM_MAIN_SRC_DIR}/utils/lit/lit.py)
+      if(EXISTS ${LLVM_MAIN_SRC_DIR}/utils/llvm-lit)
+        add_subdirectory(${LLVM_MAIN_SRC_DIR}/utils/llvm-lit utils/llvm-lit)
+      endif()
       if(NOT LLVM_UTILS_PROVIDED)
         add_subdirectory(${LLVM_MAIN_SRC_DIR}/utils/FileCheck utils/FileCheck)
         add_subdirectory(${LLVM_MAIN_SRC_DIR}/utils/count utils/count)
         add_subdirectory(${LLVM_MAIN_SRC_DIR}/utils/not utils/not)
         set(LLVM_UTILS_PROVIDED ON)
         set(CLANG_TEST_DEPS FileCheck count not)
       endif()
       set(UNITTEST_DIR ${LLVM_MAIN_SRC_DIR}/utils/unittest)